1. Field of the Invention
This invention pertains generally to video encoding and decoding, and more particularly to determination of a chroma quantization parameter within video encoders and decoders.
2. Description of Related Art
The communication and storage of videos in an efficient manner requires coding mechanisms for reducing spatial and temporal redundancies. Although a number of coding techniques exist, ongoing efforts are directed at increasing efficiencies of these enCOder/DECoders (codecs) which respectively compress and decompress video data streams. The purpose of codecs is to reduce the size of digital video frames in order to speed up transmission and save storage space. Video coding advances made over the years have collectively contributed to the high levels of coding efficiency provided by state-of-the-art codecs. It is desired, however, that coding be performed at still higher efficiencies to further decrease video bit rates.
The latest of these developing coding standards is referred to as High Efficiency Video Coding (HEVC), from the Joint Collaborative Team on Video Coding (JCT-VC), which is a joint effort of the MPEG and VCEG standardization committees. HEVC employs Coding Unit (CU) structure, whose main difference from a macroblock structure (e.g., in previous MPEG-2 or AVC codecs) is that instead of a fixed size (e.g., 16×16), the size can vary up to 128×128. One Coding Tree Unit (CTU) represents both flat area and busy area, whereby providing a single QP value for one CTU is insufficient for obtaining high levels of subjective quality. Accordingly, HEVC partitions the CTU into Coding-Units (CU), each of which are represented by their own QPs which can differ from one CU to another.
The current Chroma QP derivation process in HEVC (e.g., HM 5.0) replicates that of the H.264/AVC specification as shown in Table 1. For QP values in the range of 0 to 29 a linear relationship (QPC=QPY) is followed, whereas a nonlinear relationship is followed for higher QP values. Chroma QP also saturates at a maximum value of 39 without any consideration of the color format that may be used. It will be noted that this table actually defines the relationship between Luma and Chroma at different quality levels.
However, limiting Chroma QP in the range of [0, 39] has several disadvantages.